1. Field of the Invention
The present invention relates to an optical communication field using a Fabry-Perot laser, and particularly to a method for maintaining wavelength-locking of a Fabry-Perot laser regardless of a change of external temperature even though a temperature controller is not used, and a WDM light source using the method.
2. Description of the Related Art
In general, wavelength division multiplexing (WDM) passive optical networks (PONs) provide a very-high-speed broadband communication service using a specific wavelength allocated to each subscriber. Therefore, it is possible not only to assure a privacy protection in a communication service, but also to easily expand either a separate communication service required by each subscriber or a communication capacity. Moreover, it is also possible to expand the number of subscribers through an additional specific wavelength, which is scheduled to be allocated to a new subscriber. In spite of these advantages, due to a necessity for an additional wavelength stabilizing circuit for stabilizing both a light source having a particular oscillation wavelength and this oscillation wavelength at a central office (CO) and each subscriber unit, a high economical burden is imposed on each subscriber, so that the WDM PONs has not been commercialized up to date. Thus, to implement the WDM PONs, it is essential to develop an economical WDM light source.
As this economical WDM light source, there have been proposed a distributed feedback (DFB) laser array, a multi-frequency laser (MFL), a spectrum-spliced light source, a wavelength-locked Fabry-Perot laser for wavelength-locking with incoherent light, and so forth.
However, the DFB laser array and the MFL are produced through a complicated process, and are expensive devices in which their own light sources essentially require precise wavelength selectivity and wavelength stabilization to perform WDM.
The spectrum-spliced light source, which has been actively studied recently, performs spectrum-splicing of an optical signal of wide bandwidth using an optical filter or a waveguide grating router (WGR), so that it can provide many wavelength-divided channels. Thus, it does neither need a light source for a particular oscillation wavelength nor equipment for wavelength stabilization. As this spectrum-spliced light source, there have been proposed a light emitting diode (LED), a super-luminescent diode (SLD), a Fabry-Perot (FP) laser, a fiber amplifier light source, an ultra high frequency (UHF) optical pulse light source, and so forth.
However, the LED and the SLD, which have been proposed as the spectrum-spliced light source, are inexpensive and have a wide optical bandwidth, but have a narrow modulation bandwidth and a low power, so that they are more suitable for a light source for an upstream signal having a slow modulation speed rather than for an downstream. The FP laser is an inexpensive high power device, but has a narrow band width, so that it is impossible to provide many wavelength-divided channels. The FP laser has a disadvantage in that its degradation caused by a mode partition noise is serious when it modulates and transmits spectrum-spliced signals at a high speed. The UHF pulse light source has a very wide spectrum band of a light source and a coherency, but has a low stability of oscillated spectrum and a pulse width of several picoseconds at most, so that it is difficult to implement the UHF pulse light source.
Instead of these light sources, a spectrum-sliced fiber amplifier light source has been proposed, which is capable of providing many high power channels wavelength-divided by spectrum division of ASE (Amplified Spontaneous Emission) generated from a fiber amplifier. However, this spectrum-sliced fiber amplifier light source must additionally employ an expensive external modulator such as a Litium Niobate (LiNbO3) modulator so that channels transmit different data from each other.
Meanwhile, the wavelength-locked Fabry-Perot laser performs spectrum-splicing of an optical signal of wide bandwidth using an optical filter or an arrayed waveguide grating (AWG), in which the optical signal is generated from an incoherent light source such as an LED or a fiber amplifier light source. The spectrum-sliced optical signal is injected into a Fabry-Perot laser, and is outputted as a wavelength-locked signal. The wavelength-locked signal is used for transmission at the wavelength-locked Fabry-Perot laser. When a spectrum-spliced signal having higher power than a predetermined level is injected into the Fabry-Perot laser, the Fabry-Perot laser generates and outputs only light having a wavelength harmonized with that of the spectrum-spliced signal. The wavelength-locked Fabry-Perot laser wavelength-locked with this incoherent light perform a direct modulation according to a data signal, so that it can transmit data in a more economical manner.
FIG. 1 is a schematic view for explaining a wavelength-locked phenomenon of a general Fabry-Perot laser. In FIG. 1, of reference numerals, 10 represents an optical spectrum of a general Fabry-Perot laser 40, 20 represents an optical spectrum of external incoherent light inputted into the Fabry-Perot laser 40, 30 represents an optical spectrum of a wavelength-locked Fabry-Perot laser, which is wavelength-locked with the inputted external incoherent light.
Referring to FIG. 1, the Fabry-Perot laser 40 provides a plurality of oscillation modes, which are spaced apart from each other at a uniform wavelength interval centering around one wavelength according to a resonance wavelength of a laser diode and a gain characteristic of a manufacturing material, unlike a distributed feedback (DFB) laser outputting a single wavelength. Thus, when coherent light or incoherent light 20 is inputted from outside, some of the oscillation modes which are mismatched with a wavelength of injection light are suppressed, but the others 30 which are matched with a wavelength of injection light are amplified and outputted.
However, to output a wavelength-locked signal, which is appropriate for high-speed and long-distance transmission, the Fabry-Perot laser must inject a high power optical signal having a wide bandwidth. In addition, when an external temperature is not controlled, its variation causes a mode of the Fabry-Perot laser to be varied. Thus, the Fabry-Perot laser has its wavelength mismatched with a wavelength of the injected spectrum-spliced signal, so that it gets free from a wavelength-locked phenomenon. As a result, the Fabry-Perot laser can be no longer used as a WDM light source. This results from the fact that a wavelength of the Fabry-Perot laser has a rate of change of about 0.1 nm/° C., while a wavelength of the AWG used to perform spectrum-splicing of injection light has a rate of change of about 0.01 nm/° C. Therefore, it is impossible to avoid changing a spectral overlap between the injection light and the oscillation modes of the Fabry-Perot laser according to a change of temperature.
FIG. 2 shows changes of wavelength of external injection light and of output light of a Fabry-Perot laser according to a change of ambient temperature. In FIG. 2a, an ambient temperature is T0. As T0 is sequentially increased by an increment of ΔT in FIGS. 2b, 2c and 2d in that order, it is shown that an oscillation wavelength of the Fabry-Perot laser undergoes a red shift. Here, it can be seen from FIGS. 2a and 2d that an oscillation mode exists within a line width of 3 dB of the injection light and is wavelength-locked (indicated by a thick arrow). By contrast, it can be seen from FIGS. 2b and 2c that no oscillation mode exists within a line width of 3 dB of the injection light and is wavelength-locked.
Thus, a TEC (Thermo-Electric Cooler) controller in using the wavelength-locked Fabry-Perot laser as a WDM light source.
FIG. 3 shows a structure of the conventional Fabry-Perot laser mounted with a temperature controller.
Referring to FIG. 3, the conventional Fabry-Perot laser comprises a TEC controller 31, a thermistor 32, and a TEC 34. The TEC controller 31 senses a temperature of the Fabry-Perot laser 33 via the thermistor 32 and adjusts the temperature of the Fabry-Perot laser 33 using the TEC 34.
However, in the conventional Fabry-Perot laser, the thermistor and the TEC are coupled to the Fabry-Perot laser, so that a packing cost is increased. Further, the TEC controller is additionally mounted to the Fabry-Perot laser, so that the whole price is increased. Consequently, a WDM PON places a high economical burden on subscribers, so that it has not yet been commercialized.